The present invention relates to fuses in general, and particularly to a current-limiting, time-delay, knife blade fuse.
A current-limiting time delay fuse 10 employs a built-in delay that allows temporary and harmless inrush currents to pass without the fuse being opened, but which is designed to open in response to a sustained overload and short circuit currents. Such a dual-element fuse is used in circuits subjected to temporary inrush current transients, such as motor starting currents, to provide both high performance short-circuit current protection and time-delay overload current protection.
One conventional type of such a fuse 10, depicted in FIG. 1, comprises a body which includes an electrically insulative tube 12 formed for example of glass reinforced polyester, a pair of copper knife blade terminals 14 connected to respective brass end plates 16, and a pair of steel end caps or ferrules 18. The end caps 18 are attached to the tube 12 by screws 20 (or rivets) to close the ends of the tube and retain the end plates 16. Each terminal 14 projects through a slit 24 formed in a radial portion 15 of a respective end cap 18, and is supported or attached to the tube 12 by a flat pin or roll pin (not shown) extending through the terminal.
Alternatively, as shown in FIGS. 2 and 3, the terminals 14A could be brazed to thick end bells 16A which are inserted into respective ends of the tube 12A such that radial holes 26A formed in each end bell 16A become aligned with respective radial holes 28A formed in the tube 12A. Cylindrical drive pins 30A would be force-fit through respective pairs of holes 26A, 28A to secure the end bells to the tube.
Disposed within a cavity 32 formed by the tube 12 are fuse elements. Preferably, two types of fuse elements 34, 36 are provided, namely, an overcurrent trigger mechanism 34 and a short circuit interrupting fusible element 36. There is at least one of each type of fuse element. The cavity 32 is filled with an arc-quenching filler material 33 such as quartz sand.
Each overcurrent trigger mechanism 34 includes an alloy solder 38 for series-connecting the mechanism 34 to one of the fuse elements 36, a trigger 40, a coil compression spring 42 surrounding the trigger 40, an absorber 44 surrounding the spring 42, a heater element 46, and an insulator 48. The trigger mechanism 34 utilizes stored energy of the spring 42 to break the current in the event of low level overcurrents or overloads, and will hold an overload that is five times greater than the ampere rating of the fuse for a minimum time, e.g., about ten seconds.
Each short circuit fuse element 36 comprises a strip 50 of fusible metal, such as silver, copper, copper alloy, etc., having parallel rows 52 of perforations. Adjacently disposed perforations define therebetween current-carrying weak spots of substantially reduced cross-section designed to break in response to a short circuit overload current.
Although such fuses have performed acceptably, certain shortcomings exist. For instance, in the short circuit fuse elements 36, the strips 50 are supported only by their weak spots which provide very little strength for the fuse element while being handled during the fuse-manufacturing process. Consequently, the fuse elements 36 are susceptible to mechanical fatigue and breakage due to normal handling during manufacture, as well as due to mechanical and thermal fatigue caused by steady state and transient current load current cycling.
Heretofore, the fatigue problem due to handling has been solved by the use of special equipment, tool fixturing and procedures designed to reduce the amount of worker handling. Those measures, however, increase capital expenditures and slow the production rate.
Another shortcoming relating to a time delay current-limiting fuse, or to fuses in general, which are filled with an arc-quenching filler involves the need to plug a hole in which the filler has been introduced. In that regard, the filler is typically introduced through a hole which must be plugged or sealed, in order to retain the filler. A variety of methods of sealing or plugging have been used, such as metal drive plugs, set screws, steel balls, and metal cups, as well as adhesives and glues such as epoxy, but all suffer from various limitations. For example, drive plugs require costly fabrication machinery, set screws are also costly in that they require that the filler hole be machined to form a screw thread; balls and cups are held in place by an interference-fit and are less costly, but the interference-fit is not always reliable, whereby the balls or cups may become dislodged; adhesives are messy to apply and hard to control.
Additional shortcomings may result from the ability to provide the tubes of knife blade fuses with shorter lengths. If a fuse manufacturer is to incorporate shorter fuse tube lengths, then certain spacing requirements must be satisfied to ensure that a user can safely grip a fuse without simultaneously touching parts of the fuse which will produce an electrical shock. These spacing requirements are spelled out in the Underwriters Laboratory standards for electrical equipment that use these fuses in a covered device (i.e., disconnect switch). The spacing requirements specifically pertain to what is known as phase-to-phase and phase-to-ground distances between live and/or dead metal parts. A live metal part means a metal conductor at some voltage potential with respect to ground. A dead metal part means a metal conductor at no voltage potential with respect to ground.
In that regard, a common problem involving the application of shorter fuse tube lengths to a typical knife blade fuse design is that the longitudinal space between the live metal end caps is so short as to create spacing violations for phase-to-phase and phase-to-ground distances in existing equipment designed to specific Underwriters Laboratory standards. To overcome this spacing violation, several design approaches have been considered. One approach involved the use of heat shrink plastic wrap over the metal end caps, and another approach employed plastic end caps (e.g., see Swain U.S. Pat. No. 2,863,967). Both of those approaches proved either too expensive or impractical due to strength issues.
Yet another shortcoming involving the manufacture of shorter fuses is that in order to make the fuse body shorter the fuse blades must become longer to continue satisfying the dimensional requirements of the fuse. By making the fuse blades longer, a greater mechanical moment may be imposed during installation of the fuse. To accommodate this greater mechanical moment, a stronger mechanical system must be provided. The typical knife blade fuse depicted in FIG. 1 does not provide the necessary mechanical system to support the force exerted on the longer blade of a short-body fuse. The fuse depicted in FIGS. 2 and 3, however, will support this force because of the added strength from the pinned mechanical system to the high strength tube. However, the cost of the pinned mechanical system is too high in cost to implement for all types of knife blade fuses, because it uses a very expensive tube material (e.g., glass melamine) and the fuse must be assembled on a C-shaped metal frame which is very labor intensive.
Therefore, it would be desirable to provide a fuse of the type containing an arc-quenching filler with a more effective fill-hole plugging arrangement.
It would also be desirable to provide a short-circuit fuse element which is less susceptible to mechanical and thermal fatigue due to handling as well as due to steady state and transient load current cycling.
It would further be desirable to provide a knife blade fuse with a stronger blade arrangement that is able to withstand greater mechanical moments.
It would also be desirable to provide a knife blade fuse which provides for strong reinforcement and closure of the ends of the fuse tube while ensuring that ample phase-to-phase and phase-to-ground distances are created.